4 Answers
4

Seeing that you used an English Ale yeast, then the most likely answer is that you fermented too hot. Some restrained fruity esters are expected in nearly every ale, and the english ales definitely have them... banana is a bit unlikely one, and is usually only in styles like hefeweizens and tripels. Next time try lowering your fermentation temps to closer to 65, and use a more neutral yeast like Wyeast 1056 (American Ale) which is a cleaner, less estery yeast strain. Give that Imperial Stout some time though, higher alcohol beers can always benefit from some aging.

The ester you're experiencing is called isoamyl acetate. Esters don't contribute to hangovers. I had a banana flavor crop up in my very first batch, an extract IPA. It happened in the bottle, as best I could tell. I tasted the beer between primary and secondary, and before bottling, and there was no hint of this flavor.

Everything I read pointed toward the yeast (Safale US-05 dry) being the culprit. I couldn't find any documentation on what to expect. Was I stuck with this banana flavor or would it lessen over time? Was this just a phase and would the yeast re-metabolize this by-product?

I gutted my way through the batch, telling myself I'd do better next time.

Well, it turns out it was a phase. The IPA became excellent as it aged. The banana flavor completely disappeared (or became part of the background) and I learned a valuable lesson in patience. For being impatient, I got to drink half a batch of banana special. By the time the flavor subsided, I only had one six-pack left.

If it doesn't taste right, leave it alone. Go on to your next batch. Open a bottle every week or so to see if it's changed. By the time you get to six months, you'll have drunk half the batch in testing and can make an informed decision about whether to dump the batch.

I'm not sure about this, but I've been operating for a long time under the impression that unwanted banana (estery) aromas in general are a sign that the yeast was stressed-- either due to a hot fermentation, lack of oxygen or just plain having to work too hard. By the latter, I mean that perhaps you didn't use enough yeast for either the volume of beer or the gravity. This would play into your situation, being that your beer was an Imperial Stout and was likely rocking a pretty high gravity.

Then again, some yeasts just put off that kind of aroma. The recommendation of using a neutral yeast strain is a good one.

There are a couple of reasons your beer might have a banana aroma and tart taste. The main answer is "esters". You can get esters because of a hot fermentation (over 75˚ or so), or also because of a lack of oxygen in the wort during fermentation (low aeration).

The flavor might go down a little over time, but probably not too much.